To me, e-boarding passes seem to be a ripe opportunity for criminals to fraud airlines into flying them for free.

For instance, take the following scenario:
1. Obtain a PDF of a legitimate e-boarding pass
2. Edit details of the PDF such as flight number, date, etc using Acrobat Professional's retouch text tool
3. Check united.com to make sure the flight isn't full
4. Presto! You have a free ticket

Did airlines think of this when they opened themselves up by allowing passengers to print their own boarding passes? Don't jump to conclusions, I have NOT tried the aforementioned scheme myself, I am merely pointing something out that to me seems absurd.